Encountered the same issue as described by Sven Häfker and for sure his fix did the trick

Sven Häfker’s post:
Fix: Unable to activate worker process proxy object within the worker process
Issue:
We have encountered an issue that the Sandbox Worker Process (SPUCHostService.exe) of SharePoint 2010 wasn’t able to initialize itself on several servers. As a result Sandboxed Solutions couldn’t be used. We’ve found the following entries in the SharePoint ULS:

Fix:
After trying to fix the issue for several hours, we tried to disable the “Check for publisher’s certificate revocation”-setting for the Service Account of the Sandbox and this solved the issue. You can do this by changing the following key in the registry:

HKEY_USERS\[SID OF THE SERVICE ACCOUNT]\Software\Microsoft\Windows\CurrentVersion\WinTrust\Trust Providers\Software Publishing
The value of “State” needs to be set to “0x00023e00”.